Late years, the development of a solar cell using a semiconductor thin-film such as a silicon thin-film has been actively carried out. Heretofore, in the field of semiconductors, the μ-PCD technique has been frequently employed as a noncontact and nondestructive evaluation method for the evaluation of impurity contamination and defect (for example, a silicon wafer carrier lifetime measuring method disclosed in the following Patent Document 1).
In the μ-PCD technique, an electromagnetic wave is emitted to irradiate a semiconductor sample, thereby causing free electrons in the semiconductor sample to move (migrate) according to an electric field of the electromagnetic wave. A state of the movement is influenced by the presence of impunities, defects or the like in the semiconductor sample. Thus, an intensity of a reflected wave of the electromagnetic wave emitted to irradiate the semiconductor sample (a change in intensity of the reflected wave as compared to the emitted wave) can be treated as an index of crystalline quality of the semiconductor sample. The μ-PCD technique is designed to evaluate crystalline quality of a semiconductor sample by means of the above mechanism. In addition, the μ-PCD technique has an advantage of being able to detect (measure) the reflected wave within a significantly short period of time, in a nondestructive and noncontact manner.
However, a wavelength of an electromagnetic wave (microwave) is as long as several millimeters or more, which poses a problem of failing to evaluate crystalline quality in a small area. Moreover, in cases where a semiconductor sample has a thin thickness (is a thin-film sample), for example, when the semiconductor sample is a polycrystalline silicon sample having a thickness of about several to several ten nm, or a monocrystalline silicon sample having a thickness of several μm or less, a change in intensity of an electromagnetic wave when comparing a reflected wave to an emitted wave (a change in intensity of the reflected wave due to crystalline quality of the semiconductor sample) becomes extremely small, which poses a problem of failing to ensure sufficient measurement sensitivity, i.e., measurement accuracy. On the other hand, if an intensity of excitation light is excessively increased so as to enhance the measurement sensitivity, the sample is likely to be damaged, and a light source of the excitation light involves an increase in cost.
Therefore, the inventors of this application proposed a technique disclosed in the following Patent Document 2. This conventional technique is designed to emit excitation light having energy equal to or greater than a band gap of the above thin-film sample, to irradiate a small area of the thin-film sample, in a converging manner, thereby generating photo-excited carriers in the small area of the sample, wherein a movement of the photo-excited carriers according to an electric field of an electromagnetic wave is used, instead of the movement of free electrons. In this case, the intensity of the reflected wave which changes in response to irradiation with the excitation light is detected, so that it becomes possible to evaluate such a thin-film sample, using the detected intensity as an index representing crystalline quality in a small area (excitation light irradiation area) of the sample. Further, in this conventional technique, although a change in intensity of the reflected light is small because the excitation light irradiation area is small, and thereby becomes more susceptible to noise, an unwanted frequency component (noise) is removed by forming the excitation light as light intensity-modulated in predetermined periods and extracting a component synchronous with the intensity-modulation of the excitation light, from the detected intensity of the reflected light.
The technique disclosed in the Patent Document 2 is excellent in terms of capability to evaluate crystalline quality in a small area of a TFT or the like. However, in the conventional μ-PCD technique, in a situation where there is an electrically conductive film immediately below a semiconductor thin-film as an evaluation target, a sufficient electric field intensity cannot be obtained in the semiconductor thin-film, and an interaction of the electric field with photo-excited carriers becomes weak, which poses a problem of great difficulty in performing the measurement. Specifically, in the case where particularly low-cost amorphous silicon or microcrystalline silicon is used in a solar cell, a back (bottom) electrode is formed on a glass substrate, and then a semiconductor thin-film is formed thereon, so that the bottom electrode becomes the electrically conductive film. The same problem occurs in the filed of FPDs (Flat-Panel Displays) employing a bottom gate structure.